Peter Wells

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Peter Wells
University of California-Santa Barbara
"A kinetic lattice Monte Carlo study of post-irradiation annealing of model reactor pressure vessel steels" G. Robert Odette, Shipeng Shu, Peter Wells, Dane Morgan, Journal of Nuclear Materials Vol. 2019 312-322 Link
Significant embrittlement in reactor pressure vessel (RPV) steels can be caused by the formation of nanometer-scale MneNieSi precipitates (MNSPs) and annealing is a promising technique for reducing embrittlement. To achieve better understanding of the evolution of these precipitates at the atomic scale, a kinetic lattice Monte Carlo (KLMC) model, parameterized using CALPHAD and recent atom probe tomography (APT) data, is used to simulate post-irradiation annealing of MNSPs. The model predicts MNSP volume fractions, number densities and sizes that agree well with the experimental observations. The model also predicts that the initial structure of the precipitates may be B2 bcc phases with one sublattice occupied by Ni and the other sublattice occupied by Mn and Si, as well as shows a modest temperature dependence of the MNSP composition. The results show that the simple approach can be used to model MNSP evolution and supports that these precipitates are stable thermodynamic phases.
"CuMnNiSi precipitate evolution in irradiated reactor pressure vessel steels: Integrated Cluster Dynamics and experiments" Mahmood Mamivand, Peter Wells, Huibin Ke, G. Robert Odette, Dane Morgan, Acta Maerialia Vol. 180 2019 199-217
"Evolution of manganese–nickel–silicon-dominated phases in highly irradiated reactor pressure vessel steels" James Cole, Brandon Miller, Tim Milot, G. Robert Odette, Peter Wells, Takuya Yamamoto, Yuan Wu, Acta Materialia Vol. 80 2014 205-219 Link
Formation of a high density of Mn–Ni–Si nanoscale precipitates in irradiated Cu-free and Cu-bearing reactor pressure vessel steels could lead to severe unexpected embrittlement. Models long ago predicted that these precipitates, which are not treated in current embrittlement prediction models, would emerge only at high fluence. However, the mechanisms and variables that control Mn–Ni–Si precipitate formation, and their detailed characteristics, have not been well understood. High flux irradiations of six steels with systematic variations in Cu and Ni contents were carried out at ~295 °C to high and very high neutron fluences of ~1.3 × 1020 and ~1.1 × 1021 n cm-2. Atom probe tomography shows that significant mole fractions of Mn–Ni–Si-dominated precipitates form in the Cu-bearing steels at ~1.3 × 1020 n cm-2, while they are only beginning to develop in Cu-free steels. However, large mole fractions of these precipitates, far in excess of those found in previous studies, are observed at 1.1 × 1021 n cm-2 at all Cu contents. At the highest fluence, the precipitate mole fractions primarily depend on the alloy Ni, rather than Cu, content. The Mn–Ni–Si precipitates lead to very large increases in measured hardness, corresponding to yield strength elevations of up to almost 700 MPa.
"Fracture Toughness Characterization of Reactor Pressure Alloys from the ATR-2 Experiment" M. Sokolov, X. Chen, R.K. Nanstad, G. Robert Odette, Takuya Yamamoto, Peter Wells, Vol. 2017 Link
"Infrastructure development for radioactive materials at the NSLS-II" Eric Dooryhee, Lynne Ecker, G. Robert Odette, David Sprouster, Peter Wells, Randy Weidner, Sanjit Ghose, Theodore Novakowski, Tiberiu Stan, Nathan Almirall, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors, and Associated Equipment Vol. 880 2018 40-45 Link
The X-ray Powder Diffraction (XPD) Beamline at the National Synchrotron Light Source-II is a multipurpose instrument designed for high-resolution, high-energy X-ray scattering techniques. In this article, the capabilities, opportunities and recent developments in the characterization of radioactive materials at XPD are described. The overarching goal of this work is to provide researchers access to advanced synchrotron techniques suited to the structural characterization of materials for advanced nuclear energy systems. XPD is a new beamline providing high photon flux for X-ray Diffraction, Pair Distribution Function analysis and Small Angle X-ray Scattering. The infrastructure and software described here extend the existing capabilities at XPD to accommodate radioactive materials. Such techniques will contribute crucial information to the characterization and quantification of advanced materials for nuclear energy applications. We describe the automated radioactive sample collection capabilities and recent X-ray Diffraction and Small Angle X-ray Scattering results from neutron irradiated reactor pressure vessel steels and oxide dispersion strengthened steels.
"Microstructural changes and their effect on hardening in neutron irradiated Fe-Cr alloys" Dhriti Bhattacharyya, Takuya Yamamoto, Peter Wells, Emmanuelle Marquis, Mukesh Bachhav, Yuan Wu , Joel Davis, Alan Xu, G. Robert Odette, Journal of Nuclear Materials Vol. 519 2019 274-286 Link
"On a' precipitate composition in thermally annealed and neutron-irradiated Fe- 9-18Cr alloys" Mukesh Bachhav, Emmanuelle Marquis, G. Robert Odette, Peter Wells, Takuya Yamamoto, Elaina Reese, Journal of Nuclear Materials Vol. 500 2018 192-198 Link
Ferritic-martensitic steels are leading candidates for many nuclear energy applications. However, formation of nanoscale a' precipitates during thermal aging at temperatures above 450?°C, or during neutron irradiation at lower temperatures, makes these Fe-Cr steels susceptible to embrittlement. To complement the existing literature, a series of Fe-9 to 18 Cr alloys were neutron-irradiated at temperatures between 320 and 455?°C up to doses of 20 dpa. In addition, post-irradiation annealing treatments at 500 and 600?°C were performed on a neutron-irradiated Fe-18 Cr alloy to validate the a-a' phase boundary. The microstructures were characterized using atom probe tomography and the results were analyzed in light of the existing literature. Under neutron irradiation and thermal annealing, the measured a' concentrations ranged from ~81 to 96?at.% Cr, as influenced by temperature, precipitate size, technique artifacts, and, possibly, cascade ballistic mixing.
"On the use of charged particles to characterize precipitation in irradiated reactor pressure vessel steels with a wide range of compositions" Nathan Almirall, Peter Wells, Takuya Yamamoto, G. Robert Odette, Journal of Nuclear Materials Vol. 536 2020 Link
Nuclear reactor lifetimes may be limited by nano-scale Cu-Mn-Ni-Si precipitates (CRPs and MNSPs) that form under neutron irradiation (NI) of pressure vessel (RPV) steels, resulting in hardening and ductile to brittle transition temperature increases (embrittlement). Physical models of embrittlement must be based on characterization of precipitation as a function of the combination of metallurgical and irradiation variables. Here we focus on rapid and convenient charged particle irradiations (CPI) to both: a) compare to precipitates formed in NI; and, b) use CPI to efficiently explore precipitation in steels with a very wide range of compositions. Atom probe tomography (APT) comparisons show NI and CPI for similar bulk steel solute contents yield nearly the same precipitate compositions, albeit with some differences in their number density, size and volume fraction (f) dose (dpa) dependence. However, the overall precipitate evolutions are very similar. Advanced high Ni (>3 wt%) RPV steels, with superior unirradiated properties, were also investigated at high CPI dpa. For typical Mn contents, MNSPs have Ni16Mn6Si7 or Ni3Mn2Si phase type compositions, with f values that are close to the equilibrium phase separated values. However, in steels with very low Mn and high Ni, Ni2-3Si silicide phase type precipitate compositions are observed; and when Ni is low, the precipitate compositions are close to the MnSi phase field. Low Mn significantly reduces, but does not eliminate, precipitation in high Ni steels. A comparison of dispersed barrier model predictions with measured hardening data suggests that the Ni-Si dominated precipitates are weaker dislocation obstacles than the G phase type MNSPs
"Precipitation and hardening in irradiated low alloy steels with a wide range of Ni and Mn compositions" G. Robert Odette, Nathan Almirall, Peter Wells, Takuya Yamamoto, Acta Materialia Vol. 2019 119-128 Link
Mn-Ni-Si intermetallic precipitates (MNSPs) that are observed in some Fe-based alloys following thermal aging and irradiation are of considerable scientific and technical interest. For example, large volume fractions (f) of MNSPs form in reactor pressure vessel low alloy steels irradiated to high fluence, resulting in severe hardening induced embrittlement. Nine compositionally-tailored small heats of low Cu RPVtype steels, with an unusually wide range of dissolved Mn (0.06e1.34 at.%) and Ni (0.19e3.50 at.%) contents, were irradiated at z 290 C to z 1.4 1020 n/cm2 at an accelerated test reactor flux of z3.6 1012 n/cm2 -s (E > 1 MeV). Atom probe tomography shows Mn-Ni interactions play the dominant role in determining the MNSP f, which correlates well with irradiation hardening. The wide range of alloy compositions results in corresponding variations in precipitates chemistries that are reasonably similar to various phases in the Mn-Ni-Si projection of the Fe based quaternary. Notably, f scales with z Ni1.6Mn0.8. Thus f is modest even in advanced high 3.5 at.% Ni steels at very low Mn (Mn starvation); in this case Ni-silicide phase type compositions are observed
"Structural characterization of nanoscale intermetallic precipitates in highly neutron irradiated reactor pressure vessel steels" David Sprouster, Eric Dooryhee, John Sinsheimer, Sanjit Ghose, Peter Wells, Nathan Almirall, G. Robert Odette, Lynne Ecker, Tiberiu Stan, Scripta Materialia Vol. 113 2016 18-22 Link
Massive, thick-walled pressure vessels are permanent nuclear reactor structures that are exposed to a damaging flux of neutrons from the adjacent core. The neutrons cause embrittlement of the vessel steel that grows with dose (fluence), as manifested by an increasing ductile-to-brittle fracture transition temperature. Extending reactor life requires demonstrating that large safety margins against brittle fracture are maintained at the higher neutron fluence associated with beyond 60 years of service. Here synchrotron-based x-ray diffraction and small angle x-ray scattering measurements are used to characterize highly embrittling nm-scale Mn–Ni–Si precipitates that develop in the irradiated steels at high fluence. These precipitates lead to severe embrittlement that is not accounted for in current regulatory models. Application of the complementary techniques has, for the very first time, successfully identified the crystal structures of the nanoprecipitates, while also yielding self-consistent compositions, volume fractions and size distributions.
"Thermodynamic and kinetic modeling of Mn-Ni-Si precipitates in low-Cu reactor pressure vessel steels" Nathan Almirall, Philip Edmondson, G. Robert Odette, Peter Wells, Huibin Ke, Leland Barnard, Dane Morgan, Acta Materialia Vol. 138 2017 10-26 Link
Formation of large volume fractions of Mn-Ni-Si precipitates (MNSPs) causes excess irradiation embrittlement of reactor pressure vessel (RPV) steels at high, extended-life fluences. Thus, a new and unique, semi-empirical cluster dynamics model was developed to study the evolution of MNSPs in low-Cu RPV steels. The model is based on CALPHAD thermodynamics and radiation enhanced diffusion kinetics. The thermodynamics dictates the compositional and temperature dependence of the free energy reductions that drive precipitation. The model treats both homogeneous and heterogeneous nucleation, where the latter occurs on cascade damage, like dislocation loops. The model has only four adjustable parameters that were fit to an atom probe tomography (APT) database. The model predictions are in semi-quantitative agreement with systematic Mn, Ni and Si composition variations in alloys characterized by APT, including a sensitivity to local tip-to-tip variations even in the same steel. The model predicts that heterogeneous nucleation plays a critical role in MNSP formation in lower alloy Ni contents. Single variable assessments of compositional effects show that Ni plays a dominant role, while even small variations in irradiation temperature can have a large effect on the MNSP evolution. Within typical RPV steel ranges, Mn and Si have smaller effects. The delayed but then rapid growth of MNSPs to large volume fractions at high fluence is well predicted by the model. For purposes of illustration, the effect of MNSPs on transition temperature shifts are presented based on well-established microstructure-property and property-property models.
"Thermodynamic models of low-temperature Mn–Ni–Si precipitation in reactor pressure vessel steels" G. Robert Odette, Peter Wells, Wei Xiong, Huibin Ke, Ramanathan Krishnamurthy, Leland Barnard, Dane Morgan, MRS Communications Vol. 4 2014 101-105 Link
Large volume fractions of Mn–Ni–Si (MNS) precipitates formed in irradiated light water reactor pressure vessel (RPV) steels cause severe hardening and embrittlement at high neutron fluence. A new equilibrium thermodynamic model was developed based on the CALculation of PHAse Diagrams (CALPHAD) method using both commercial (TCAL2) and specially assembled databases to predict precipitation of these phases. Good agreement between the model predictions and experimental data suggest that equilibrium thermodynamic models provide a basis to predict terminal MNS precipitation over wider range of alloy compositions and temperatures, and can also serve as a foundation for kinetic modeling of precipitate evolution.
"Thermodynamics and kinetics of core-shell versus appendage co-precipitation morphologies: An example in the Fe-Cu-Mn-Ni-Si system" Shipeng Shu, Peter Wells, Nathan Almirall, G. Robert Odette, DD Morgan, Acta Materialia Vol. 157 2018 298-306 Link
""Late Blooming Phases in RPV Steels: High Fluence Neutron and Ion Irradiations" Collin Knight, Brandon Miller, Tim Milot, G. Robert Odette, Peter Wells, Takuya Yamamoto, NuMat 2012 October 22-25, (2012)
"Effect of Ni on Formatoin of Entermetallic Phases in Highly Irradiated Reactor Pressure Vessel Steels" G. Robert Odette, Nicolas Silva, Peter Wells, Yong Yang, TMS 2014 February 16-20, (2014)
"Late Blooming Phases in RPV Steels at High Fluence and Flux" James Cole, Collin Knight, Brandon Miller, G. Robert Odette, Peter Wells, Takuya Yamamoto, International Group of Radiation Damage Mechanisms 17th Semiannual Meeting May 19-24, (2013)
"On the Evolution of Late Blooming Phases in RPV Steels: Theoretical Foundations, Experimental Observations and Recent Insights" Nicholas Cunningham, G. Robert Odette, Peter Wells, Takuya Yamamoto, The Minerals, Metals and Materials Society 2013 Annual Meeting March 3-7, (2013)
"On the Opportunities and Challanges for Using Test REactor and Charged Particle Irradiations to Help Predict Inaccessible In-service Neutron Irradiations Effects" Peter Hosemann, Peter Wells, Takuya Yamamoto, TMS 2014 February 16-20, (2014)
"The Evolution of Late Blooming Phases from High to Very High Fluence" Collin Knight, Brandon Miller, G. Robert Odette, Peter Wells, Takuya Yamamoto, The Minerals, Metals and Materials Society 2013 Annual Meeting March 3-7, (2013)
"The Evolution of Late Blooming Phases in RPV Steels: Theoretical Foundations, Experimental Observations Recent Insights and Implications to Life Extension" Nicholas Cunningham, G. Robert Odette, Peter Wells, Takuya Yamamoto, International Group of Radiation Damage Mechanisms 17th Semiannual Meeting May 19-24, (2013)
"Thermodynamic models of low-temperature Mn-Ni-Si precipitation in reactor pressure vessel steels" G. Robert Odette, Peter Wells, Materials Research Society April 21-25, (2014) Link